1. Field of the Invention
The present invention relates generally to a wavelength division multiplexing (WDM) optical communication system, and in particular, to a method and apparatus for monitoring a wavelength division multiplexed (WDM) optical signal transmitted from the WDM optical communication system.
2. Description of the Related Art
A WDM optical communication system transmits an optical signal with a plurality of channels. Due to its high transmission efficiency and large data capacity, the WDM optical communication system is widely used for the ultra high speed Internet. Since a WDM optical signal is attenuated as its transmitting distance increases, there is a need for a method and apparatus for monitoring the characteristics of the WDM optical signal such as wavelength and intensity.
Optical signal monitoring apparatuses using a fiber Fabry-Perot filter have been popular since it is small and has a high resolution. The transmission wavelength of the fiber Fabry-Perot filter varies according to a driving voltage. When the driving voltage is applied linearly, the transmission wavelength changes non-linearly. Moreover, the optical Fabry-Perot filter experiences changes in its transmission wavelength according to its operation temperature.
FIG. 1 is a schematic view of a conventional WDM optical signal monitoring apparatus. In FIG. 1, there is shown an optical fiber 110, a Fabry-Perot filter 130, an optical detector 150, an analog-digital converter (ADC) 160, a controller 170, a digital-analog converter (DAC) 180, and a filter driver 190. During operation, an optical signal 120 including a WDM optical signal 122 and two reference lights 124,126 propagates in the optical fiber 110.
The Fabry-Perot filter 130 has a transmission wavelength that varies according to a linearly applied driving voltage 195. That is, the Fabry-Perot filter 130 passes only an optical signal with a predetermined wavelength at a predetermined driving voltage without passing optical signals with other wavelengths. As the driving voltage 195 applied to the Fabry-Perot filter 130 linearly increases, the transmission wavelength of the Fabry-Perot filter 130 also increases gradually.
The optical detector 150 converts a WDM optical signal 140 received from the Fabry-Perot filter 130 to an analog optical detection signal 155. The ADC 160 converts the analog optical detection signal 155 to a digital optical detection signal 165. The DAC 180 converts a digital driving signal 175 received from the controller 170 to an analog driving signal 185. The filter driver 190 applies a driving voltage to the Fabry-Perot filter 130 in accordance with the analog driving signal 185.
The controller 170 outputs the digital driving signal 175 and receives the digital optical detection signal 165 from the ADC 160. The controller 170 also derives a linear approximation formula from wavelengths of the two reference lights 124,126 and driving voltages corresponding to the reference wavelengths detected from the digital optical detection signal 165. The controller 170 also determines the wavelengths of the signal channels 122 using the linear approximation formula.
FIG. 2 illustrates a graph 210 showing linear approximated wavelengths and a graph 220 showing real transmission wavelengths for the fiber Fabry-Perot filter 130 of FIG. 1. As shown in FIG. 2, the real transmission wavelength of the fiber Fabry-Perot filter 130 changes non-linearly with respect to the linearly applied driving voltage 195. The conventional optical signal monitoring apparatus approximates the real transmission wavelength graph 220 to the linear graph 210. That is, a linear approximation formula is formed using the predetermined wavelengths X1,X2 that define a predetermined wavelength band to be measured, which is the wavelength of an optical signal to be measured within the predetermined wavelength band. Further, the driving voltages V1,V2 are also used, which are related to the predetermined wavelengths X1,X2. Then the real wavelength graph 220 is approximated to the linear graph 210 satisfying the linear approximation formula which is defined as:                     x        =                                                                              X                  1                                -                                  X                  2                                                                              V                  1                                -                                  V                  2                                                      ⁢                          (                              v                -                                  V                  1                                            )                                +                      X            1                                              (        1        )            
where x is a linear approximated wavelength and v is a driving voltage related with x. For example, if a predetermined optical signal is detected upon application of a third driving voltage V3 to the fiber Fabry-Perot filter 130, the predetermined optical signal is measured to have a fourth transmission wavelength X4, though, its real transmission wavelength is a third transmission wavelength X3.
As described above, the conventional optical signal monitoring apparatus using a fiber Fabry-Perot filter has a drawback in that the non-linearity of the transmission wavelength graph of the fiber Fabry-Perot filter is not appropriately compensated. That is, because the conventional optical signal monitoring apparatus is based on the assumption that the transmission wavelength graph of the fiber Fabry-Perot filter is linear, the measured wavelength of an input optical signal differs from its real wavelength.